We could turn North Korea into a radioactive parking lot in about half an hour. That's not the problem. The problem is that in the meantime we could lose one or two of our cities, and have a large zone around them where nobody could live for decades or even centuries. That could happen in South Korea too.

AND other countries might get involved--China and Russia are the ones we're most concerned about, That would make the war bigger and longer and might cost us one or two or several more cities.

We have no reason to go to war with North Korea, and vice versa. Neither country has anything to gain from a war like this even if it wins.